If you perform an online upgrade an rebalance with 1.8.1 and 2.0.2 nodes, it may fail and
produce the error. This is caused by requesting rebalance too quickly after adding a node.
To avoid this problem you should script a delay of 10 seconds
after you add a node before you request rebalance.

added to Use Online Upgrades for Couchbase Server 1.8 to Couchbase Server 2.0:

Be aware that if you perform a scripted online upgrade from 1.8.x to 2.0 you should have a 10 second delay between adding a 2.0 node to the cluster and rebalancing. If you request rebalance too soon after adding a 2.0 node, the rebalance may fail.

kzeller
added a comment - 08/May/13 5:14 PM - edited Added to 2.0.2 RN as:
If you perform an online upgrade an rebalance with 1.8.1 and 2.0.2 nodes, it may fail and
produce the error. This is caused by requesting rebalance too quickly after adding a node.
To avoid this problem you should script a delay of 10 seconds
after you add a node before you request rebalance.
added to Use Online Upgrades for Couchbase Server 1.8 to Couchbase Server 2.0:
Be aware that if you perform a scripted online upgrade from 1.8.x to 2.0 you should have a 10 second delay between adding a 2.0 node to the cluster and rebalancing. If you request rebalance too soon after adding a 2.0 node, the rebalance may fail.